This invention relates to a luminance signal/color signal separation circuit, and more particularly to a circuit for separating Y (luminance) signals and C (carrier chrominance) signals from a composite image signal e.g., in a VTR etc.
In a conventional Y/C separation circuit, a composite image signal (color bar signal) incoming to the input terminal is changed to a 1H delayed signal by a 1H delay circuit. The 1H delayed signal is subtracted from the composite image signal at the subtracter. The signal thus obtained further goes through a bandpass filter and a 1/2 amplifier and is than taken out from the C output terminal as a C signal. On the other hand, the composite image signal is delayed at .DELTA.t corresponding to the delay time of the bandpass filter at a .DELTA.t delay circuit. The C signal is subtracted from the .DELTA.t delayed signal at a subtracter and is then taken out from the Y output terminal as a Y signal.
However, such an image signal processing circuit has the problems that a thin color portion called a half amplitude portion occurs in the C signal, whereby color at vertical transition portions of a pictorial image is thinned or color fringe is shifted downward in the displayed picture, and that dot crawl due to C signal crosstalk occurs in the Y signal, resulting in considerably degraded picture quality. A further problem with this image signal processing circuit is that vertical resolution lowers in the reproduction of fine detail, etc. and thus shading in which color is changed to gray occurs, lacking clearness.
The truth table of the comb filter in the conventional circuit is shown in Table 1. In this Table, an asterisk indicates the half amplitude or the color shift of the C signal (dot crawl in the case of the Y signal), and a double asterisk indicates that a signal is considered as the C signal as a whole.
TABLE 1 ______________________________________ a b c ______________________________________ 0 0 0 1 0 1/2* 0 1 -1/2* 1 1 0 1 -1 1** -1 1 -1** ______________________________________